Though thus more general in application, one of the important uses of the invention is in the field of computer or related display screen systems, such as cathode-ray tube displays (or LCD, LED, electroluminescent or other electro-optical displays or the like); and it is therefore to this exemplary use that the invention will hereinafter be described as an important illustration.
A modern computer typically presents its user with such a display screen on which may be presented descriptions or pictoral representations of various choices or selections which the user may make. In many cases, the quickest, easiest, and most intuitive way for the user to respond is by physically finger-touching the areas of the screen which show the desired selections.
To allow this, the computer must be equipped with an input device which permits the program on which it is operating to determine the fact and location of such touch events. For present purposes, any input device of this sort will be termed a "touch screen".
A desirable touch screen input device should be inexpensive, rugged, reliable, and sufficiently accurate. It is also very desirable that a single model work with a wide range of different display devices, and that it be susceptible to easy field installation by untrained users, either on new or on existing equipment.
Unfortunately, existing touch screens, such as those later described, are of relatively low manufacture volume and thus very expensive by the standards of their natural market, being therefore precluded in major usage from integration at the time a display is manufactured. In addition, they require great effort, expense, and manufacturing expertise to retrofit. Since each model is more-or-less unique to a specific screen geometry, different models must be made in great profusion, or would-be users must be restricted in their display choice. For a combination of functional and cosmetic reasons, thus, certain prior art touch screens are indeed built into the display device, such as a cathode-ray tube, at initial manufacture (though expensive, due to low volume), and others require an awkward retrofit (also expensive). Such prior touch screens, moreover, are closely tied to the design of the display device with which they are to be used, and must be provided in a profusion of different types to find wide application. Many, furthermore, have inherently expensive sensor structures tightly constrained by the geometry, compatibility, and packaging constraints of the associated display, so that sensor structures cannot often be optimized for cost.
Turning to such prior art techniques for determining touch location on a cathode-ray tube or similar display screen, they involve some combination of distributing sensors around the periphery of, or over the surface of, the actual displaying surface or screen. Such known methods employing force sensing to locate the point at which a force is applied to a surface generally embody three or more force sensors placed in a plane, but not allowed to lie along a single line. The axis of sensitivity of each is oriented perpendicular to this plane, and the outputs of the sensors are used to compute the location of contact forces which are applied in this same plane. If and when the contacted surface is allowed to depart from this plane, the unpredictable tangential components of the contact force must necessarily cause errors in the reported location. If the contact surface lies far from the plane of the sensors (or is severely non-planar), prior methods are ineffective.
Specifically, a first system of this nature is adapted for the front portion of cathode-ray tube screen displays, being provided with various additions to enable touch localization, including both resistive and capacitive sensing technologies, in which an extra sensor plate is applied over the face of the display screen. The plate bears one or two layers of transparent conductor patterns which develop and convey touch location information to conductors at the edge of the overlay plate. While efforts are made to keep all components transparent, losses in practice are sufficient substantially to reduce image brightness and clarity. Examples of such touch screen sensors may be found in U.S. Pat. Nos. 4,198,539; 4,293,734; 4,353,552; 4,371,746; 4,806,709; and 4,821,029.
A second approach involves surface acoustic wave (SAW) technology in which a glass overlay plate carries acoustic energy generated, redirected, and sensed by transducer and reflector means disposed about the periphery. Touching the plate damps this energy in a manner particular to the contact location, as described, for example, in Eleographics 1987 flier "Surface Acoustic Wave".
Another technique has involved a planar force sensing technology in which piezoelectric force transducers support a glass overlay plate, attaching it to a mounting. The intersection of a finger-touch thrust line with the transducer plane occurs at a point which is associated with a specific ratio of transducer outputs, allowing the position of this point within the plane to be computed. When curved, phosphor-bearing screen surfaces must necessarily deviate from the plane, creating a particular form of parallax error in which the user, expecting response at a particular point, instead actually receives response at another point. Sensor techniques and signal processing suitable for such an approach are described, for example, in U.S. Pat. Nos. 4,340,777; 4,355,202 (and prior art strain gauge sensors described therein including U.S. Pat. No. 3,657,475 and "One-Point Touch Input of Vector Information for Computer Displays," C. Herot et al., Computer Graphics, Vol, 12, No. 3, pp. 210-216); and U.S. Pat. No. 4,675,569.
Still another approach uses planar force-sensing technology in which steel beam springs with strain gauge transducers constitute force sensors bearing the entire weight of, for example, the cathode-ray tube assembly. This technology avoids the image degradation of an overlay plate, but at the cost of requiring greater sensor dynamic range and problems of rejection of stray signals from sway and vibration. Its function is otherwise substantially identical to the above-described piezoelectric system. U.S. Pat. Nos. 4,918,262 and 5,038,142 describe such a system, citing, also, earlier piezoelectric and related sensors.
Infrared light technology has also been proposed in which many separate beams travelling from emitters to detectors define a plane. When the user's finger (or other probe of sufficient width) crosses this plane, the identity of interrupted beams locates the "touch". Again, a transverse component to the touch motion can lead to a parallax error in which response at the expected location is replaced by response at an unexpected location. Parallax errors for this technology tend to be particularly severe, since the response surface cannot be positioned to intersect the phosphor surface, nor be shaped to conform to it. Additionally, such apparatus may require obtrusive bezels. An example of such a system is described in pages 12-44 of a text entitled "Caroll Touch", which also summarizes the before-described resistive-capacitive sensor overlay systems, surface acoustic wave systems and piezoelectric systems, as well.
Each of the above methods has an effective response surface which, unfortunately, fails to be coincident with the active surface of the display, leading to the universal prior performance imperfection of parallax.
The before-described resistance, capacitance and acoustic plate sensors have a response surface which conforms to the actual physical surface of touch contact, such lying visually about 1/2 inch in front of the phosphor surface in the case of a cathode-ray tube display. An operator whose eye is somewhat to the side, will therefore perceive an error in the touch system response unless touching a surface point that lies directly over the desired target point, rather than the target point itself.
The piezoelectric and other planar force-sensing systems above-described, on the other hand, do not actually report an actual location of surface contact, but rather provide what may be called an "indicated point" on a "virtual response surface". The indicated point is at the intersection of the thrust line and the plane of the force sensors. For the described infrared beam system, such an indicated point is where the finger breaks the plane of the infrared beams. Since the glowing phosphors are not located in such plane, the virtual surface does not correspond to anything visible or intuitive, making the parallax error of these devices particularly troublesome.
Underlying the present invention, however, is the discovery of a novel method of and apparatus for enabling a wide variety of cathode-ray tube or other screen display systems, as in computers, monitors and other video systems and the like, to be placed upon or in touch with a common, universal force-sensing platform, the sensors of which are thus external to the plane of the display screen and remote even from the display equipment itself, but nonetheless provide a novel three-dimensional force locating technique for forces, such as the finger-touching of the display screen, while obviating all of the above-described limitations and disadvantages of the prior art techniques, including the total elimination of parallax.
Other distinguishing features of the invention from the above-described and other prior art approaches will be more fully addressed hereinafter.